The microstructural evolution, mechanical properties, fracture behaviors and age hardening mechanism in 1Cr15Ni36W3Ti subjected to various thermal treatments involving a preceding solution at 1080 °C and three distinct aging schedules were investigated. Upon aging treated at 870 °C and then 700 °C, discrete globular MC carbides combined with dispersive and fine γ′ particles were formed at grain boundaries and in the γ matrix, respectively, which is beneficial in resisting the propagation of grain-boundary microcracks and dramatically enhancing yield and ultimate strengths. Rather, the second aging strategy conducted at 730 °C after solution resulted in relatively coarse γ′ precipitates and nearly continuous intergranular MC carbides, while the third aging schedule carried out firstly at 780 °C and then 730 °C led to a dual size of γ′ phase and also a nearly continuous distribution of the grain-boundary MC carbides. Moreover, γ′ denuded zones on both sides of grain boundaries were formed in the latter two aging treatments, which were severer in the third age. The relatively coarse γ′ phase and the appearance of precipitate-free zones dramatically lowered the strength, whereas the nearly continuous intergranular carbides with γ′ denuded zones alongside accelerated the propagation of intergranular micro-cracks. Dislocation shearing was dominated. The γ′ coherency strengthening, the introduction of complex stacking faults in the γ′ phase arising from Shockley partial dislocations shearing and the elimination of γ’ denuded zones were responsible for the highest tensile strength when undergoing the first-type age.